1. Field of the Invention
This invention relates to an electrode substrate and a recording medium. It also relates to a method of manufacturing such an electrode substrate and a recording medium.
2. Related Background Art
In recent years, massive efforts have been paid for developing new materials to be used for memories as such materials are deemed to take a vital role for the electronic industry of manufacturing computers, computer-related devices and audio-visual devices such as video discs.
While properties that memory materials are required to have may vary depending on the application, they normally include
(1) a highly dense and large memory capacity,
(2) a high response speed for data recording/reproduction,
(3) a low power consumption rate and
(4) a high productivity at low cost.
While semiconductor memories and magnetic memories made of a magnetic or semiconductor material have been in the main stream, low cost and high density recording media such as optical memories using organic thin film made of an organic pigment or a photopolymer are currently on the scene as a result of the remarkable development in the field of laser technologies.
Meanwhile, thanks to the recent development of scanning tunneling microscopes (hereinafter referred to as STM) that allow a direct observation of the electronic structure of surface atoms of a conductor material [G. Binnig et al., Phys. Rev. Lett., 49, 57 (1982)], it is now possible to observe a real spatial image of a specimen with an enhanced level of resolution regardless if the specimen is crystalline or non-crystalline. An STM provides an advantage of low power consumption rate that makes the specimen free from power-related damages in addition to the fact that it can be operated in the atmosphere to observe various specimens and hence provides a wide variety of applications.
The STM utilizes the phenomenon that a tunneling current flows between the metal probe of the STM and the electroconductive specimen when they are brought close to each other until they are separated only by about 1 nm, while applying a voltage to them.
The tunneling current is highly sensitive to changes in the distance separating them. Therefore, various information can be obtained on the real spatial arrangement of the entire electron cloud by operating the scanning probe so as to maintain the tunneling current to a constant level. The intraplanar resolution of an STM is typically about 0.1 nm.
Thus, an ultra-high density data recording/reproduction on the order of the size of an atom (on the order of sub-nanometer) will be possible on the basis of the principle of STM.
For instance, a data recording/reproducing apparatus disclosed in Japanese Patent Application Laid-Open No. 61-80536 utilizes an electron beam to remove particles of atoms adsorbed on the surface of a recording medium in order to write data onto and read data from it by means of an STM.
Methods have been proposed for recording/reproducing data on a material exhibiting memory effects for voltage-current switching characteristics such as a thin film of a n electron type organic compound or a chalcogen compound by means of an STM (see, inter alia, Japanese Patent Applications Laid-Open Nos. 63-161552 and 63-161553).
With any of such methods, it is possible to record data as densely as 1012 bits/cm2 when the recording bit size is 10 nm.
FIG. 1 of the accompanying drawings schematically illustrates the configuration of a known information processing apparatus utilizing the STM technology. This apparatus will be described briefly below.
Referring to FIG. 1, there are shown a substrate 11, a metal electrode layer 12 and a recording layer 13. There are also shown an XY stage 201, a probe 202, a probe support member 203, a linear actuator 204 for driving the probe in the direction of the Z-axis and a pulse voltage circuit 207.
Reference numeral 301 denotes an amplifier for detecting the tunneling current flowing from the probe 202 to the electrode layer 12 by way of the recording layer 13. Reference numeral 302 denotes a logarithmic compressor for converting the change in the tunneling current into a value proportional to the gap between the probe 202 and the recording layer 13. Reference numeral 303 denotes a low-pass filter for extracting any surface unevenness components of the recording layer 13.
Otherwise, there are shown an error amplifier 304 for detecting the difference between the reference voltage Vref and the output of the low-pass filter 303, a driver 305 for driving the Z-axis linear actuator 204 and a drive circuit 306 for positionally controlling the XY stage 201 by means of X- and Y-axis linear actuators 205 and 206. Reference numeral 307 denotes a high-pass filter for separating data components.
FIG. 2 of the accompanying drawings schematically illustrates a probe 202 to be used with a known recording medium.
Referring now to FIG. 2, there are shown data bits 401 stored in the recording layer 13 and crystal grains 402 produced when the electrode layer 12 is formed on the substrate 11. The crystal grains have a size of about 30 to 50 nm if the electrode layer 12 is formed by means of commonly used techniques such as vacuum evaporation or sputtering.
The gap between the probe 202 and the recording layer 13 can be held to a constant value by means of the circuit shown in FIG. 1. More specifically, the tunneling current flowing between the probe 202 and the recording layer 13 is detected and, after passing through the logarithmic compressor 302 and the low-pass filter 303, compared with a reference voltage. Then, the Z-axis linear actuator 204 supporting the probe 202 is driven to reduce the difference between the detected value and the reference value to zero and thereby maintain the distance between the probe 202 and the recording layer 13 to a constant value.
Then, the XY stage 201 is driven to make the probe 202 move along the surface of the recording medium so that the data stored in the recording layer 13 can be detected at point b by separating the high frequency component of the signal obtained at point a in FIG. 1.
FIG. 3 of the accompanying drawings is a graph showing the signal intensity spectrum relative to the frequency of the signal obtained at point a in FIG. 1. Note that the signal portion below f0 represents the mild undulations of the surface of the recording medium due to warps and distortions of the substrate 11 and the part of the signal at and around f1 represents the surface roughness of the recording layer 13 mainly due to crystal grains 402 produced at the time of forming the electrode material and the signal portion at f2 represents the carrier wave component of the recorded data. Reference numeral 403 denotes the data signal band.
Reference symbol f3 denotes the part of the signal for which the atomic and molecular arrangement of the recording layer 13 is responsible.
However, a known recording medium having a configuration as described above is typically accompanied by the following problems.
For a high density recording to be done by exploiting the high resolution of an STM, the data signal band 403 should be found between f1 and f3.
Then, a high-pass filter 307 relevant to cut-off frequency fc is used to separate the data component of the signal.
However, the data signal band 403 lies on the outskirt of the signal component represented by f1 mainly due to the fact that crystal grains 402 of the electrode layer 12 is responsible for the signal component f1 and the size of the crystal grains 402 that is about 30 to 50 nm is close to the recorded data size and the bit interval which are about 1 to 10 nm.
A net consequence of this is a low S/N ratio for data reproduction and a high error rate for data reading.
Therefore, it is an object of the present invention to provide an electrode substrate and a recording medium showing a high S/N ratio and adapted to high speed data reproduction by solving the problems of the prior art. Another object of the present invention is to provide a method of manufacturing such an electrode substrate and a recording medium.
According to an aspect of the invention, the above first object is achieved by providing a substrate comprising a metal electrode layer and/or a recording layer, wherein said metal electrode layer and/or said recording layer have a smooth surface area with a surface roughness of less than 1 nm by more than 1 xcexcm2.
According to another aspect of the invention, there is provided a method of manufacturing an electrode substrate having a metal electrode layer comprising steps of:
forming a metal electrode layer on a first substrate having a smooth surface; and peeling said first substrate off said metal electrode layer, transferring the smooth surface profile of said first substrate to the surface of said metal electrode layer.
According to still another aspect of the invention, there is provided a method of manufacturing a recording medium having a metal electrode layer comprising a step of:
forming a recording layer on a metal electrode layer of an electrode substrate prepared by the above manufacturing method.
According to still another aspect of the invention, there is also provided a method of manufacturing a recording medium comprising steps of:
forming a recording layer on a first substrate having a smooth surface; and
peeling said first substrate off said recording layer, transferring the smooth surface profile of said first substrate to the surface of said recording layer.